I unlocked my Pearl from ATT and moved to Tmobile. I installed the Tmobile O/S (deleted vendor.xml) and the Pearl shows the Tmobile version of the software. But the ATT themes are still on the device by default and no tmobile themes. Also, the Instant Messaging application says that it is not a TMobile Pearl so I cannot use it.

It seems as if the Tmobile OS install possibly did not work properly but I can't seem to force it to reload the OS.

Once an AT&T Pearl, always an AT&T Pearl no matter what version of the OS you put on it. Your ATT Pearl has a vendor code on it that can't be changed. AT&T's vendor code is 102. For the other themes take a look at this:

The AT&T themes are present because it's an AT&T pearl and has a vendor code of 102. It will always load the software as if it's an AT&T Pearl. That's why you have the AT&T stuff. Edit the blackberry.alx as described in that FAQ and you'll have all the themes. You can edit it so as to add just the themes you want, as well.

Its not a good IM app anyway and it uses sms text messaging so you need a text plan. I recommend JiveTalk. It's a one time $29.99 fee but it's a good working app and supports all the major IM services.